Техническое чтение для энергетиков. Бухарова Г.П. - 32 стр.

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34
they may not pass through the same phase at the same instant of time. For example,
two currents may be such that, although their frequency may be the same, their phase
at a particular instant of time may be different. One may pass through its maximum
value at the instant when the other has a zero value, or some other value not its
maximum value; the two currents, etc., are then said to have a phase difference, this
being quoted in degrees. I one current has its maximum value at the same time that
the other is zero, the two currents are said to be in quadrature; they have a phase
difference of 90°. Phase difference, because it is constant in a circuit where steady
conditions obtain, is much more important in a.c. work than the actual phase which
varies from instant to instant.
Phase difference is illustrated in Fig. 3,
where two currents are shown not in phase.
The phase difference is measured by the
distance between the points where the two
graphs cross the base line in the same
direction. This distance is measured in
electrical degrees, the scale being obtained
by considering the distance corresponding to
one complete cycle as 360°. The current that
is ahead in phase is said to lead the other
current, while this current is said to lag
behind the first current. Similarly, when
considering two voltages, one is said to lead
and the other to lag. Again, a voltage may
lead the current that it produces, the current lagging behind the voltage.
Maximum and Average Values.– An alternating current varies in strength from
instant to instant, and reaches a maximum value twice in every cycle, once in each
direction. This maximum value is called the crest value. (See Fig. 2).
The average value, taken throughout a complete cycle, is equal to zero, since there is
as much current flowing in one direction as there is flowing, in the other direction.
Considering a half-cycle, however, the average value has a definite positive value.
This can be evaluated by measuring the lengths of a number of equidistant ordinates
and taking their average value. In the case of a sine wave, this average value is found
to be equal to
π
2
or 0.687 times the maximum value but is not, however, the value
read on an ordinary ammeter, nor is it the value commonly used to denominate the
value of an A.C.
** the ideal aimed at is that of the sine wave идеалом, к которому стремятся, является
синусоидальная волна; aimed at = which is aimed at; that – заменитель ранее упомянутого
существительного
PRODUCTION ОF THREE-PHASE CURRENTS
In an ordinary a. c. circuit the current goes through all its phases in succession,
but at any particular instant the current has only one phase. In the three-phase system
Fig. 3. Illustrating Phase Difference

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